Adding the open-source-pdks repository under `third_party`.

Goal is to include common information about open source PDKs into the
GF180MCU PDK documentation. This includes;
 * File naming information -
   https://github.com/google/open-source-pdks/issues/7
 * Versioning scheme -
   https://github.com/google/open-source-pdks/issues/8
 * Other open source PDKs that exist.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>